JCB Hydromax Hydrogen Speed Record Hits 406mph

A hydrogen combustion engine is almost identical in terms of its mechanical structure to a petrol or diesel engine. The pistons still go up and down in the cylinders, the crankshaft still turns the linear motion into rotation, and the driveshaft still passes on the power to the wheels. The only difference lies in what is fed into the cylinder and what is exhausted from it. Instead of injecting petrol vapour and then igniting it with a spark, or squeezing the diesel until it catches fire on its own, the hydrogen engine injects hydrogen gas and burns it, either by means of a spark plug or by carefully adjusting the compression timing.

The chemistry involved is so clear it's almost embarrassing. In its ideal state, the combustion of hydrogen follows the equation 2H2 + O2 to 2H2O: two hydrogen molecules react with one oxygen molecule to give water and at the same time release energy in the form of heat. Since there is no carbon in the fuel there is no carbon dioxide produced as a result of burning it. The problem, and it is a real one, is that hydrogen burns at a higher temperature than petrol or diesel, and this heat causes the nitrogen and oxygen in the surrounding air to react and form nitrogen oxides, or NOx, a real pollutant which engineers have to deal with by means of exhaust after-treatment or by carefully controlling the air-fuel ratio.

That is the way in which Hydromax differs from a fuel cell vehicle, something that anyone examining energy transfer at A-level should understand. A fuel cell produces electricity by combining hydrogen and oxygen in an electrochemical reaction, without involving combustion, and this electricity then directly drives an electric motor. It is generally more efficient at turning the chemical energy stored in hydrogen into useful motion since it avoids the thermodynamic losses which are present in all heat engines - the same kind of losses that mean a petrol engine can only convert about one third of the energy in its fuel into motion, the rest being lost as waste heat, a limitation explained by the Carnot efficiency, which sets the maximum amount of work that any heat engine can obtain from a temperature difference. On the other hand, a combustion engine is generally able to deliver power more rapidly and at higher outputs than the current fuel cell stacks, which is one of the reasons why Hydromax was able to break both the 303mph fuel cell record and the previous combustion record.

Another key characteristic of hydrogen is that it takes up a lot of space. As a gas, hydrogen has a very low energy density by volume, even though its energy density by mass is extremely high, about three times that of petrol. In order to carry sufficient hydrogen for a high-speed journey, Hydromax's tanks must probably store the gas at a very high pressure, the pressure usually being 350 or 700 bar in hydrogen vehicles, the gas being held in carbon-fibre-wrapped cylinders built to withstand both the pressure and the vibrations caused by a car travelling at 400mph. The safe engineering of such tanks is in fact a major problem in the field of materials science: metals that are exposed to hydrogen over a long period can suffer from hydrogen embrittlement, a condition in which hydrogen atoms seep into the metal lattice and cause it to be more likely to crack, a phenomenon that is closely examined by engineers involved in hydrogen infrastructure, whether it be for cars or for pipelines.

There's also the question of aerodynamics, since the land speed record comes down more to the shape of the vehicle than to the amount of fuel used. At speeds above 400mph, air resistance is by far the most significant force acting on the car. Since drag is proportional to the square of the speed, increasing the speed from 200 to 400mph does not result in the aerodynamic resistance being doubled; rather it increases by about four times. The Hydromax has a long and slim shape measuring 32 feet and this reduces the frontal area while smoothing the flow of air around the body, thus minimising drag as much as possible without at the same time losing the necessary downforce to keep the car stable on the salt at high speed. Engineers who are constructing these record-breaking cars have to take into account the practical restrictions of the Bonneville track: if too much downforce is used, the rolling resistance goes up and there is a risk of the tyres digging into the salt crust.

When students are considering different specialisms in engineering, Hydromax involves a number of fields such as the thermodynamics of internal combustion, the chemistry of hydrogen combustion and NOx formation, the materials science of high-pressure gas storage, and the fluid dynamics of high-speed aerodynamics. It is rare for a single project to involve so many of the disciplines in mechanical and automotive engineering.